Water bottle cap



FIG. 1 is a side, right front perspective view of a water bottle cap showing my new design;

FIG. 2 is a side view thereof;

FIG. 3 is a front view thereof;

FIG. 4 is a top plan view thereof;

FIG. 5 is a rear view thereof; and,

FIG. 6 is a bottom plan view thereof. 

The ornamental design for a water bottle cap, as shown and described. 